ANTimiCROBIAL AGENTS AND CHEMOTHERAPY, Oct. 1977, P. 551-554 Copyright © 1977 American Society for Microbiology
Vol. 12, No. 4 Printed in U.S.A.
In Vitro Activity of Spectinomycin Against Recent Urinary Tract Isolates ROBERT J. FASS* AND RICHARD B. PRIOR Division of Infectious Diseases, Department of Medicine, The Ohio State University College of Medicine, Columbus, Ohio 43210 Received for publication 13 April 1977
The susceptibilities to spectinomycin of 303 recent urinary tract isolates were determined and compared to the susceptibilities of those strains to ampicillin, tetracycline, and gentamicin. Based on minimal inhibitory concentrations, 84% of Escherichia coli, Klebsiella, and Enterobacter, 31% of other Enterobacteriaceae, 7% of Staphylococcus aureus and Streptococcus (including enterococci), and 0% of Pseudomonas aeruginosa were susceptible to concentrations of spectinomycin that are easily surpassed in serum (c32 ,ug/ml); 90% of all organisms tested other than P. aeruginosa were susceptible to concentrations that are easily surpassed in urine (s 128 ,g/ml). Spectinomycin was active against more isolates than either ampicillin or tetracycline but against fewer isolates than gentamicin. Disk diffusion susceptibility tests did not reliably distinguish susceptible from resistant isolates with any of the four antibiotics studied.
Spectinomycin hydrochloride is similar to the aminoglycosides because it is an aminocyclitol antibiotic that has a broad spectrum of in vitro activity against both gram-positive and gramnegative bacteria (4, 7, 16). Although it has been used successfully to treat a variety of infections (1, 5, 14), it is generally less active in vitro than the aminoglycosides (4) and has been used primarily for the treatment of uncomplicated anogenital gonorrhea (7). Spectinomycin is unlike the aminoglycosides, however, because it has been given in high doses (up to 8 g/ day) intramuscularly and intravenously without oto- or nephrotoxicity (9, 10). Peak serum concentrations greater than 100 gg/ml and peak urine concentrations of approximately 9,000 ,ug/ml were observed after 2-g doses (15), and such concentrations, particularly those observed in urine, would be inhibitory for most urinary pathogens (4, 7, 16). This study was undertaken to determine the percentage of recent urinary isolates that are inhibited by surpassable serum and urine concentrations of spectinomycin and to determine if a standardized disk diffusion test (13) could be used to distinguish susceptible from resistant organisms. For comparative purposes similar studies were performed with ampicillin, tetracycline, and gentamicin. Three hundred three urinary isolates were randomly selected from the positive cultures obtained by the bacteriology laboratory in University Hospital, Columbus, Ohio, during the period January through December 1976.
Minimal inhibitory concentrations (MICs) were deternined in duplicate by a microdilution method (3), using Trypticase soy broth (Baltimore Biological Laboratory), an inoculum of 105 to 106 colony-forming units per ml, and serial twofold dilutions of spectinomycin, ampicillin, tetracycline, and gentamicin such that final concentrations tested ranged from 1,024 to 1 Ag/ml. With ampicillin, additional dilutions from 0.5 to 0.03 Ag/ml were tested against Staphylococcus aureus. A control strain, Escherichia coli ATCC 25922, was tested on each day that MIC tests were performed; results showed variations no greater than one dilution step from the statistical mode MICs. The criteria for defining susceptibility to each antibiotic are shown in Table 1. For ampicillin, tetracycline, and gentamicin, breakpoints for susceptibility to surpassable serum concentrations were based on those recommended by the National Committee for Clinical Laboratory Standards (13). For spectinomycin, the breakpoint for susceptibility to surpassable serum concentrations was that recommended by Washington and Yu (16). Breakpoints for susceptibility to surpassable urine concentrations were chosen after consideration of a variety of pharmacological data and the recommendations of an international collaborative study
(2). The cumulative percentages of urinary isolates inhibited by various concentrations of spectinomycin are shown in Fig. 1. MICs were similar to those previously reported (4, 7, 16); 551
ANTJMiCRO3. AGzNTs CHEMOTHZR.
most strains of E. coli, Klebsiella, and Enterobacter were susceptible to concentrations surpassable in serum (c32 ILg/mIl), most strains of other Enterobacteriaceae, Streptococcus, and S. aureus were susceptible to concentrations surpassable in urine (s128 pg/ml), and strains of Pseudomonas aeruginosa were consistently resistant. The percentages of strains that were susceptible to spectinomycin, ampicillin, tetracycline, and gentamicin are compared in Table 2. Spectinomycin was active against more isolates than either ampicillin or tetracycline, although ampicillin was more consistently active against Proteus mirabilis and Streptococcus, and tetracycline was more consistently active against S. aureus in concentrations surpassable in serum and against P. aeruginosa in concentrations surpassable in urine. Spectinomycin was less consistently active than gentamicin; concentrations of gentamicin surpassable in serum inhibited 75% of the strains tested, and concentrations surpassable in urine inhibited 100% of the strains tested other than enterococci. Disk difffusion tests (13) for each isolate were TABLz 1. MIC breakpoints for determining susceptibility to four antibiotics
done simultaneously with the MIC determinations. Disk contents were 100 pg of spectinomycin, 30 ug of tetracycline, and 10 pg of ampicillin and gentamicin. The results of disk diffusion tests and corresponding MICs were plotted on a scatter graph for each antibiotic. Zones of inhibition were plotted on a linear scale as the radius squared (x axis), and MICs were plotted on a linear log2 scale (y axis). The "ideal" leastsquares line of regression (expressed as y = mx + b, where m was the slope and b was the y intercept) and the correlation coefficient (r) were calculated. When the zone of inhibition was indeterminate (i.e., s6 mm) or the MIC was indeterminate (i.e., l1 or >1024 ,ug/ml) for a given organism-antibiotic combination, the data for that combination were not included in the calculation of the corresponding line of regression. For ampicillin, the data for S. aureus were also excluded from the calculations. The zones of inhibition around spectinomycin disk and the corresponding MICs for each ofthe 303 urinary isolates are shown in Fig. 2. The
MIC (;Lgml) 40
Susceptible Serum Urine
Spectinomycin Ampicillin Tetracycline Gentamicin a MIC s 0.2 aureus.
s32 s8a s4 s4
s128 s 128a s64 564
.256 256 .128
Ag/ml indicates susceptibility for S.
256 512 12 (Ahml) OF 3PETOW
FIG. 1. Cumulative percentages of urinary isolates inhibited by various concentrations ofspectinomycin.
Percentage of urine isolates susceptible to four antibiotics in concentrations surpassable in serum and urine
Susceptible (%) No. tested
E. coli Klebsiella
Enterobacter Serratia P. mirabilis Proteus (+ indole) Citrobacter P. aeruginosa S. aureus
100 20 22 8 40 20 13 20 10 10
84 85 82 50 30 20 38 0 20 10
Ampicilhn Urine 84 72 75 0 14 45 0 38 93 95 0 40 15 92
Tetracycline Serum 45
85 45 0 3 15 85
68 100 91 50 18 40 92
Gentamicin Urine 100 100 100 100 100 38 100 63 100 60 100 100 100
100 63 90 60 100 0 100 100
group A or D)
VOL. 12, 1977
,jg/ml LOG2 n
i1024 'I10 z 0
y--0.0335x +9.04 tt
r -.-0.77 n-
+*X +D_dX x
2 SI 0 o ADIUSA 2 (mm2)
DIAMETER (mnm) 6 8
INIIBlrlON - 100AL9 SPECTINOMYCIN DISC
FIG. 2. Regression line of spectinomycin; radius squared of the zone of inhibition versus MIC. Symbols: *, E. coli; 0, Klebsiella; *, Enterobacter; O, Serratia; x, P. mirabilis; t, indole plus Proteus; tt, Citrobacter; A, P. aeruginosa; A, S. aureus; V, (3-Streptococcus; V, Enterococcus.
relationship of zones of inhibition and corresponding MICs was curvilinear, and there was great variability in zone diameters around the calculated ideal line of regression. All zone diameters, including those suggested by Washington and Yu (16), that were considered as potential breakpoints for distinguishing susceptible from resistant strains seemed unsatisfactory; they resulted in a high percentage of resistant strains being falsely classified as susceptible and/or a high percentage of susceptible strains being falsely classified as resistant. Based on correlation coefficients for calculated regression lines and the clustering of results near MIC breakpoints for defining susceptibility to both achievable serum and achievable urine antimicrobial concentrations, disk diffusion susceptibility tests with ampicillin (y = -0.038Lx + 6.64; r = -0.77; n = 138) and gentamicin (y = -0.030a + 5.66; r = -0.75; n = 231) were similar to that for spectinomycin in their unreliability for distinguishing susceptible from resistant strains. With tetracycline (y = -0.0420x + 7.30; r = -0.89; n = 195), the correlation coefficient was more acceptable, primarily because of the biphasic pattern of distribution of MICs for the organisms tested, but the scatter of results around breakpoints still made the test unreliable for accurately predicting susceptibility as defined by MICs. The relative importance of antimicrobial concentrations achievable in serum and urine in treating bacteriuric patients has been controversial, but sufficient evidence exists to indicate that cure depends more upon concentrations in urine than in serum (6, 8, 11, 12). Based on the MIC data from the current study, it
would be reasonable to conclude that many bacteriuric patients with pathogens resistant to drugs such as ampicillin and tetracycline could be successfully treated with spectinomycin rather than an aminoglycoside such as gentamicin. Before the usefulness of spectinomycin in treating urinary tract infections could be established, however, clinical studies would be necessary to document its in vivo efficacy, particularly since early studies (5, 14) indicated that the administration of low doses (1 to 2.4 g/ day) of spectinomycin to some bacteriuric patients resulted in the development of bacterial resistance in vivo. We thank Beth A. Bailey and Sally Z. Hausman for their excellent technical assistance. This study was supported by a grant from The Upjohn Co., Kalamazoo, Mich. LITERATURE CITED 1. Barry, J. M., and R. Koch. 1963. Actinospectacin serum levels and clinical data, p. 538-542. Antimicrob. Agents Chemother. 1962. 2. Ericsson, H. M., and J. C. Sherris. 1971. Antibiotic sensitivity testing. Report of an international collaborative study. Acta Pathol. Microbiol. Scand. Sect. B Suppl. 217. 3. Fass, R. J., C. A. Rotilie, and R. B. Prior. 1974. Interaction of clindamycin and gentamicin in vitro. Antimicrob. Agents Chemother. 6:582-587. 4. Karney, W., K. K. Holmes, and M. Turek. 1973. Comparison of five aminocyclitol antibiotics in vitro against Enterobacteriaceae and Pseudomonas. Antimicrob. Agents Chemother. 3:338-342. 5. Lindemeyer, R. I., M. Turck, and R. G. Petersdorf. 1962. An evaluation of a new antibiotic, actinospectacin, in infections of the urinary tract. Am. J. Med. Sci. 244:478-483. 6. McCabe, W. R., and G. G. Jackson. 1965. Treatment of pyelonephritis. Bacterial drug and host factors in success or failure among 252 patients. N. Engl. J.
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